The Essence of "Females" and "Males"

In discussing these findings, we will refer to "females" and "males" for clarity, as these terms are more recognizable to the public. However, it's essential to clarify that this terminology refers to individuals assigned female at birth (AFAB) and those assigned male at birth (AMAB).

The distinction between sex and gender is significant in scientific discourse. Here’s a breakdown:

• "Females" and "males" denote biological sex — the inherent differences between the two.
• "Women" and "men" encompass gender, which includes complex psychosocial identities and societal expectations related to each sex.

It's important to recognize that being AFAB or AMAB does not strictly correlate with possessing the "XX" or "XY" genotype, respectively. The biological spectrum is more complex than that.

The Evolutionary Perspective on Variability

Charles Darwin noted in his 1871 work, The Descent of Man, that males generally exhibit greater variability in traits compared to females. This variability can be illustrated through the example of height: if you were to line up 100 men and 100 women by height, you'd observe a broader range of heights among men.

This variability can also be seen in other physical traits such as body size, aerobic capacity, and measures of strength. Cognitive abilities and behavioral traits, such as sleep patterns and activity levels, also tend to show more variation in males.

The implications of this variability extend to mate selection, with certain traits being proxies for resource accessibility. For instance, taller or stronger individuals might be perceived as having better survival prospects.

Exploring Energy Expenditure Variability

This brings us to key questions addressed in the study: Is energy expenditure — the number of calories burned throughout the day — also more variable in males than in females? And if so, does this influence mate selection?

The study utilized a database that measured adults' energy expenditure using "doubly-labeled water." Researchers examined three primary metrics:

1. Basal Energy Expenditure (BEE): The minimum calories burned to maintain life.
2. Activity Energy Expenditure (AEE): Calories burned through all activities, including fidgeting and exercise.
3. Total Energy Expenditure (TEE): The total calories burned daily, accounting for all body processes.

Findings indicated that males exhibited significantly greater variability across all three measures compared to females. This variability could be attributed to differences in height and lean mass, which are known to affect metabolic rates.

Behavioral traits also come into play, with higher variability in physical activity levels among males correlating with greater AEE and TEE. Interestingly, BEE, which measures calories at rest, also showed substantial variability in males.

Why Are Females Less Variable?

A key factor in human biology is the differing reproductive costs between sexes. While males can theoretically reproduce with numerous females with minimal investment, females face significant biological and energetic costs associated with pregnancy and childbirth.

This evolutionary dynamic means that traits favoring female reproductive success tend to be conserved in the gene pool, while males can afford a broader range of traits as long as they can reproduce.

In conclusion, such studies deepen our understanding of human evolution and the intricate differences between the sexes.